1. Field of the Invention
The present invention relates to vehicle recording systems, such as aircraft cockpit voice recorders; and more particularly to backup power supplies which enable such recording systems to operate in the absence of primary power in the vehicle.
2. Description of the Related Art
The United States Federal Aviation Administration (FAA) requires that most commercial aircraft be equipped with a crash-survivable cockpit voice recorder (CVR). This recorder collects vital flight information that provides evidence for the reconstruction and analysis in the event of an accident. The CVR is usually powered from an electrical bus of the aircraft.
During an accident, the aircraft may lose electrical power that normally supplies the CVR, thereby causing this device to terminate recording critical information long before the culmination of the accident. As a result, the FAA regulations now mandate that all aircraft, that are required to carry CVRs, be fitted with an independent power source that is located with the recorder and that activates automatically to power the recorder for ten minutes plus or minus one minute of operation, whenever the primary aircraft power is unavailable, either due to manual shutdown or electrical system failure.
Existing backup power supplies store electrical energy in either a battery or a capacitor bank. Capacitors tend to be preferred as being more able to be repeatedly cycled through charge and deep discharge operations without significantly affecting the life of that storage device.
Nevertheless, the capacitance of large capacitor banks decreases over time due to the applied voltage level, as well as temperature to which they are subjected. The voltage level has a significant impact on the rate at which the capacitance decreases. For example, an individual capacitor that is rated for 2.5 volts can experience a 20% capacitance reduction, if continuously charged to 2.5 volts for two years. Then again, that same capacitor, if only charged to 2.1 volts can last twenty years at that same temperature. Therefore, it is desirable to maintain the voltage on the capacitor at as low a level as possible to provide sufficient backup power to meet the FAA ten minute requirement.